It is generally known that in urban areas where the transmission and reception via satellite of audio signals is limited or not available, land repeaters have to be used in the systems of transmission and reception of audio signals so as to guarantee maximum reliability. For this reason, amplifiers of audio signals used in the systems of transmission and reception must be capable of using audio signals with a wide volume range, from −90 dBm to 0 dBm, without any degradation of the signal/noise S/(N+I) ratio, where S indicates the power of the input signal, N indicates the noise and I indicates the interference.
To avoid the saturation of the cascade stages in the case of multistage amplifiers, the amplifiers must be capable of reducing the gain when the power of the input signal increases. To keep a good signal/noise ratio the amplifier must present a low noise pattern when the input signal is very low and a good linearity when the input signal is very high, keeping a good input coupling to avoid problems due to the standing waves.
A low noise amplifier of this type is described in U.S. Pat. No. 6,600,371. The amplifier comprises a first circuit block capable of converting into current a first voltage signal that is input to the amplifier, and a second circuit block adapted to divide the current coming from the first block. The second block is controlled by a second voltage signal and the first and second circuit blocks provide a variable voltage gain to the amplifier. The amplifier also comprises first and second resistors and a feedback network; the first resistor is connected to a first output terminal of the second block and to a supply voltage, and the second resistor is connected between the first output terminal of the second block and a second output terminal of the second block. The feedback network is coupled to the first terminal and to the first circuit block, and the second terminal is coupled to at least one output terminal of the low noise amplifier. The low noise amplifier permits good input coupling to be obtained, but nevertheless does not assist in an increase in the linearity when the gain decreases. In addition there is a limited reduction of the gain due to the ratio between the resistors.
Another circuit structure for a low noise amplifier provides, in addition to the circuitry of the amplifier previously described, a transistor that is coupled with the first circuit block, constituted by bipolar transistors, to modulate the emitter degeneration resistor so as to reduce the gain. With a circuit structure of this kind there is an increase in the input linearity when the gain is reduced but the input coupling is lost.